With increasing age, the ability of the immune system to protect against new antigenic challenges or to control chronic infections erodes. One of the immediate implications of immunosenescence is a decline in the response to prophylactic vaccinations increasing the susceptibility to viral and bacterial infections. Our studies have shown that the mechanisms underlying the defective adaptive immune response change with age. After the age of 75 years, the naTve CD4 T cell compartment is small and its diversity is severely contracted, essentially abating the ability to mount a T cell response to new vaccines. Before that age, the repertoire diversity of the naTve CD4 compartment is intact, and we postulate that cell-intrinsic defects compromise immunity. Compared to young adults, we have identified two signatures that are characteristic of naTve CD4 T cell responses in 60 to 74 year-old healthy individuals, failed induction of type I interferon-stimulated genes (ISG) in CD4 T cells and sustained increases in cytoplasmic zinc that leads to the overinduction of metallothioneins, further dysregulation of cytoplasmic zinc homeostasis and changes in the cellular redox state. In contrast to CD4 T cells, naTve CD8 T cells disappear more rapidly and CD8 memory T cells develop a signature dominated by the expression of negative regulatory receptors. In the current application, we propose to examine the mechanisms underlying these signatures as well as their implications for T cell differentiation and function. We use in vitro systems with T cells from individuals of different ages and an in vivo system of individuals undergoing yellow fever vaccination. In Specific Aim 1, we will examine the hypothesis that a defect in interferon signaling is responsible for the failure to induce ISG. We will use gene profiling to determine the role of interferon stimulation in T cell differentiation, and we will explore stimulation condition to minimize this defect. In Specific Aim 2, we will determine the impact of the age dependent zinc-MT signature on naive CD4 T cell responses and examine whether the change in cytoplasmic zinc homeostasis is amenable to stimulation conditions. Specific Aim 3 will examine the hypothesis that the transcription of negative regulatory receptors is epigenetically controlled and that their increased expression with age relates to global DMA CpG demethylation.